Chapter 6: Life in Transition Part 2
Our world 363 Million Years Ago, it is now the Late Devonian and life on Earth is struggling. The Kellwasser Event that occurred 9 million years earlier saw the extinction of 75% of all genera, now the great reefs that once covered Earth in droves during the Mid Devonian are a shell of their former selves, while some reefs remain they are now quite rare, and it won't be until the Mesozoic Era that they will return to their former glory and the reefs of that time will be stone corals, not the horn corals and stromatoporoid sponges of this time period. While normally life would have already recovered by this point multiple smaller extinction events have occurred, something that has kept life on edge. Despite this some creatures have survived and they continue to struggle on.
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Here in what is now Cleveland Ohio, we find ourselves in a shallow open sea. These waters are home to a variety of creatures, take this Ctenacanthus a type of chondrichthyan, related to modern day sharks. Ctenacanthus is made up of multiple different species with the largest approaching great white sharks in size, this particular species, however, is quite a bit smaller being around the size of a bull shark, despite this it is still a very effective hunter. This one has picked up the smell of a carcass and it quickly swims off in the direction of it, Ctenacanthus are opportunists and they won't turn down an easy meal. The Ctenacanthus quickly finds what it is looking for, a dead placoderm that measures up to 20 feet.
The placoderm is a Titanichthys, despite its large size Titanichthys is a gentle giant that feeds on plankton and other microscopic organisms, it is unknown how this one died, but it should make for a good meal regardless. The Ctenacanthus begins to probe for a weak point and once it finds a good spot it begins to feed. The slicing teeth of Ctenacanthus allow it to grip its prey before thrashing its head around to tear off bite-sized chunks of flesh. As it feeds it is soon joined by more Ctenacanthus who begin to join in on the feast, it is quite rare that one gets to enjoy a meal by themselves in these waters. As the waters become more chaotic, the feeding frenzy begins to attract the attention of a much larger predator, the apex predator of these waters.
Emerging from the depths is a massive Dunklesoteus at 20 feet in length it is the size of a modern day great white shark. Dunkleosteus is a type of placoderm and a relative of Titanichthys, but unlike its filter feeding relative Dunkleosteus is a ferocious carnivore. The presence of the Dunkleosteus alone puts the Ctenacanthus on edge, but luckily for them it's more focused on the feast right in front of it. Using its powerful jaws the Dunkleosteus begins to feed. Dunkleosteus has very different feeding habits from the Ctenacanthus, for one it lacks true teeth instead it has sharp scissor like plates, secondly is its bite, Dunkleosteus has enormously powerful jaws that can crack the toughest armor, allowing it to feed on the tougher parts of a carcass.
The Dunkleosteus quickly begins to show this off as it bites off chunks of the carcass, cracking right through the armor to get to the flesh. After eating its fill the Dunkleosteus begins to swim off, allowing the Ctenacanthus to resume feeding. After swimming a short distance, the Dunkleosteus regurgitates anything that it can't digest. The mass of waste sinks to the sea floor. The Dunkleosteus then resumes its territorial patrol.
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Elsewhere we find a very different group of placoderms searching for food, a shoal of Titanichthys. Titanichthys are a world wide genus and they thrive in sun lit shallow waters. They spend most of their day lazily swimming through the water. As they swim they open their mouths sucking in water, they then filter out microscopic organisms in the water which they consume. Despite the small size of these organisms, they are so abundant that they are in fact the most efficient source of food for large animals, in the future the largest animals in Earth's history, the baleen whales will evolve to feed on microscopic organisms like these.
The advantage of such large size is that Titanichthys have nothing to fear from most predators, it helps that they have thick plated armor on top of their large size of protect them, the only thing they fear are full grown Dunkleosteus, for they are the only creature with strong enough jaws to break through their defenses.
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The ocean is not the only place where life continues its fight for survival, on land life continues to struggle on. While life in the sea was heavily affected by the Kellwasser Event, life on land and in fresh water has fared quite a bit better. While some creatures like the fishapods didn't make it, other animals have. In fact one such creature is currently crawling out of the river and onto dry land. Meet Hynerpeton, a type of tetrapodomorph.
Hynerpeton is one of many new types of tetrapodomorph, notably these creatures are quite a bit more advanced than something like Tiktaalik. For one thing while Tiktaalik was stuck with fins, Hynerpeton has full-on legs with webbed feet to boot, something that gives it an edge on dry land. Hynerpeton's shoulder muscles are also very powerful, even compared to other tetrapodomorphs of this time aiding it in moving around. Despite these adaptations Hynerpeton still spends much of its time in the water, the webbed feet and paddle like tail still make it a more proficient at swimming than walking, still its adaptations are important ones and future Tetrapodomorphs will continue to build on this foundation. This particular Hynerpeton has come ashore to bask and rest.
This will keep it safe from predators, at only around 2 feet Hynerpeton is very much in danger of attack should it run into the bigger fish of this river. The area that Hynerpeton inhabits will one day become Pennsylvania, at this time a coastal delta with many rivers and streams cutting through the forest. To the west it is bordered by a shallow sea where creatures like Dunkleosteus thrive, to the east it is bordered by a mountain range being pushed up as continental plates collide. The area itself is covered with Archaeopteris trees, plants like these were unaffected by the Kellwasser Event and they still thrive in large numbers across floodplains like this one. In the shade of an Archaeopteris tree the Hynerpeton continues to rest, the small invertebrates that thrive in the leaf litter are no threat to it.
After a short time the Hynerpeton rises and heads back into the water to hunt. It swims gracefully through the water using its feet to push branches out of the way, it isn't long before it spots a Groenlandaspis, a small type of benthic placoderm, similar to Bothriolepis. It begins to swim towards its prey, it then grabs it in its jaws. The Hynerpeton then thrashes its head around killing the Groenlandaspis. It then swims back to shore.
It emerges from the water with its victim in tow. Hynerpeton prefer to feed on land when they can, this helps to keep them safe from the large fish in their environment. Using its jaws the Hynerpeton rips off chunks of flesh and swallows them whole, the Groenlandapis will make a good meal for now.
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Rivers like the one that this Hynerpeton calls home are teeming with life for a variety of fish thrive here. Ctenacanthus is one such fish for they are at home in both fresh and saltwater. Ctenacanthus use these rivers for a few reasons food is plentiful for one with fish like Groenlandaspis being a great source of protein, the river also has far less competition than the open ocean does as larger fish like Dunkleosteus and the larger species of Ctenacanthus can't survive in freshwater environments. This also makes the rivers a perfect spot to give birth as the young won't have to face as many large and dangerous predators. Of course this river isn't totally safe and there is one fish that can prey on Ctenacanthus here.
The Ctenacanthus suddenly flees as the king of the river begins to arrive, meet Hyneria lindae, a type of lobe finned fish. At up to 11 feet in length Hyneria isn't quite as big as a Dunkleosteus, but it is still a formidable threat all the same. Hyneria is an absolutely ferocious apex predator, its massive jaws are filled with teeth that can cut right through flesh, which when combined with powerful jaws allows Hyneria to hunt just about any animal here. Luckily for the denizens of the river, this Hyneria is not currently in hunting mode, instead it is simply exploring its home. The powerful fins of Hyneria allow it to push through the thick vegetation of the river, a massive advantage and one that the tetrapodomorphs also used.
Of course Hyneria's fins are not as powerful as those of the tetrapodomorphs, for Hyneria is restricted to the water. Despite this Hyneria is still a formidable predator and it is one that most fish would be wise to avoid when it is in hunting mode.
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Elsewhere we find ourselves back in the shallow sea. Swimming through the waves is the Dunkleosteus from before and it is in hunting mode. Being as large as it is, a full grown Dunkleosteus can basically hunt anything they want, but their favorite prey item is the Titanichthys. Titanichthys are large enough to provide a substantial meal for a Dunkleosteus and with their powerful jaws Dunkleosteus is one of the few animals that can punch through their armor. Before it can hunt a Titanichthys, however, it must first find one.
It spends about 2 hours combing the ocean looking for a Titanichthys and at long last it finds a shoal of them. The Dunkleosteus begins to move in slowly, it must proceed with caution, while Titanichthys lack any visible weapons they can be a formidable opponent. Their large bulk means that they can dish out vicious injuries with their powerful tails, plus they have been known to gang up on unlucky Dunkleosteus and ram them to death. The Dunkleosteus scans the group looking for a weak individual to target. After a tense moment the Dunkleosteus targets a Titanichthys and begins the attack.
Rather than panic as expected, however, the shoal huddles close together, leaving the Dunkleosteus unable to truly launch an attack. Things are about to get worse for the Dunkleosteus, one of the Titanichthys suddenly breaks off and rams the Dunkleosteus right in the mid section stunning the beast. Emboldened, the rest of the Titanichthys also launch an attack with another one ramming the Dunkleosteus. Realizing that it is now in danger, Dunkleosteus begins to flee. It swims as fast as it can to escape the now angered Titanichthys.
Luckily for the Dunkleosteus the Titanichthys quickly give up the pursuit. Soon the Titanichthys calm down, now safe from danger as they resume feeding, today they have successfully fended off danger and now they can resume their favorite pastime, eating.
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Elsewhere the Dunkleosteus begins to recover from the onslaught. Today's hunt was a disaster and had it been more unlucky the Titanichthys could've killed it. Even the most powerful predators fail more hunts than they succeed and this Dunkleosteus will have to try its luck elsewhere.
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Back in the delta we find the Hynerpeton relaxing on the bank. His relaxation doesn't last long as soon the sun's rays begin to beat down on him. Hynerpeton similarly to modern amphibians have skin that must stay moist or else they will dry out, because of this Hynerpeton must stick close to the water. The Hynerpeton quickly climbs into the water. He begins to swim around, he is joined by a few other Hynerpeton.
They have all likely come to the water for the same reason, unfortunately for them they are now facing danger from the aquatic predators that call this place home. The Hyneria from earlier has spotted the group and it begins to swim towards them. While Hynerpeton are effective swimmers, they can't even begin to match a Hyneria in open water, their best option is to head to the shore. The group spreads out as they begin to flee, the hungry Hyneria begins to look for a target. It spots our male and heads right for him.
The male swims quickly towards the shore, the Hyneria in hot pursuit, he finally reaches the shallows where the Hyneria can't reach him. The Hyneria turns around and heads back into the deeper water where the other Hynerpeton are. It spots a female who is slower than the rest and with a quick burst of speed it snatches her up in its jaws. The teeth of the Hyneria skewer the unlucky Hynerpeton, as blood fills the water. The Hyneria then gulps down its catch.
From the shore the male Hynerpeton watches the carnage unfold. After everything dies down he can relax again. He must enter the water again soon or risk drying out in the heat, but luckily with its hunger satisfied for now the Hyneria is unlikely to attack him. After a short wait the male Hynerpeton reenters the river. Luckily with the Hyneria now gone he can relax.
Something big is going to happen soon, an event that is arguably the most important for this male's life, breeding season. Once it begins the male must compete with other males for females, it will be tough as finding a mate can be challenging, but he must make an attempt.
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Back in the shallow sea, we find the Dunkleosteus once again on the prowl. It moves slowly through the water attempting to find anything that can serve as a meal. As the Dunkleosteus swims it slides the plates of its lower and bottom jaw together creating a loud grinding sound, by doing this the Dunkleosteus ensures that its main weapons will remain sharpened. After a while the Dunkleosteus finally stumbles upon something, a lone Titanichthys. It is hard to say why this Titanichthys is alone, but one thing is certain, it is in danger of an attack.
Seeing an opportunity the Dunkleosteus gets into position. Then with an impressive burst of speed the Dunkleosteus rushes in and clamps down in the Titanichthys midsection. The Titanichthys thrashes around in surprise, as the Dunkleosteus tears off a chunk of flesh. The Dunkleosteus swallows the chunk of meat and then goes in again delivering another powerful bite. Blood begins to fill the water as the Titanichthys continue to thrash around.
It isn't long until the Titanichthys bleeds out. The Dunkleosteus then begins to feed on the carcass. The thrashes of the Titanichthys has attracted the attention of some Ctenacanthus, however, they know better than to attempt to get a bite right now, they will have to bide their time for now. After a while the Dunkleosteus finishes its meal and then swims off allowing the Ctenacanthus to dig in.
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Meanwhile back in the delta, night has fallen and out of the water comes the male Hynerpeton. With the sun's rays no longer beating down on him, he can begin to move around on land for a longer period of time. This also makes the night the perfect time for the male to begin his arduous trek. He quickly moves into the forest passing through low lying ferns as he does, this spooks small arthropods hiding in the leaf litter. As he continues his journey, the giant Archaeopteris loom above him, without any large herbivores to eat them, these trees can spread far and wide.
Within just a few million years most of the Earth's landmasses will be covered in great forests like this one. The male Hynerpeton meanwhile continues on with his trek, he is approaching his destination. In the distance the first rays of sunlight begin to peek over the horizon, the sign that the day has begun. While this would be a good thing for us, its bad news for the male Hynerpeton, he must hurry before the sun rises too high and begins to truly beat down on his sensitive skin. Luckily for the male after just a few minutes he finds his destination.
The male has arrived at a secluded pool and he is not alone. Male and female Hynerpetons have swarmed this place. The pool is where the Hynerpeton breed, here their young will be safe from predators and they can grow up in relative safety. Here male Hynerpeton must compete with other males to attract females, these affairs are not violent ones, however, instead the males show off their brightly colored throats to the females lifting their heads straight up into the air. Brighter throats are preferred by the females as they show the fertility of the males, strategies like these will be used by many animals in the future and they will only grow more elaborate in later species.
Our male immediately begins to try his luck with one of the females. He walks right up to her and begins to show his throat to her. His throat is brightly colored, a sign of his fertility and strength. She quickly becomes receptive to his advances and the pair walk over to the water's edge. The female then begins to lay her eggs in earnest, while the male fertilizes them.
Hynerpeton eggs lack the protective layer of reptiles and birds, as a result they must be laid in the water, yet another thing that ties them back to the water. After the pair finishes they break off from each other as the male looks for other females to court.
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As the day drags on more eggs are laid and fertilized. Soon as the sun begins to set the Hynerpeton start to move on and by nightfall they have all gone. They leave behind their still developing young. In a few weeks the young will hatch out, but for now all is quiet at the breeding pool.
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Back in the shallow sea we find a Dunkleosteus on patrol, this Dunkleosteus is not the one we have been following, however, instead it is a rogue individual swimming in the same waters our male rules. It doesn't take long for our male to notice his presence and soon they meet face to face. Dunkleosteus are solitary animals with very strict territorial boundaries and competition for space is often violent with cannibalism being a common result of such battles. The two males begin to circle each other, waiting for the perfect moment to strike. Timing is key if one strikes too early their attack may be avoided which often leads to a chunk being ripped off, waiting too long often leads to the same result.
Luckily for our male this is not his first rodeo and he has perfected his timing. Our male strikes and clamps down on his rival tearing a massive chunk of flesh right off. The rival male hopelessly thrashes about as his blood fills the water, coloring it red. As he sinks to the bottom the ever present Ctenacanthus are quick to swarm upon him feeding on him while he is still alive. Normally the resident male would feed on his rival's carcass, but with the Titanichthys still digesting in his stomach, he simply doesn't have the appetite.
He simply swims off leaving the Ctenacanthus to their feast. Having successfully defended his territory he can once again relax.
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Elsewhere we find ourselves back at the breeding pool. It has been a few weeks since the Hynerpeton mated and laid their eggs and now the young are beginning to hatch. In the breeding pool young Hynerpeton force their way out of the eggs that they have been encased in for the last few weeks. Like modern day amphibians, the young Hynerpeton are known as tadpoles and they are very different from the adults. The young lack lungs and are fish like in shape lacking the legs of the adult Hynerpeton, because of this they are fully aquatic unlike the adults.
As the weeks pass the young will gradually transition into adults. For now they will spend their days in the shallow pool feeding on small bits of vegetation. As they approach their adult size they will move onto dry land and head towards the many waterways that dot the region.
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Elsewhere we find the male Hynerpeton patrolling through the water of a small pond. As the sun begins to set he crawls onto land, the sun's rays vanishing under the horizon. Tetrapodomorphs like Hynerpeton have proven to be very successful throughout the late Devonian as they have only continued to diversify as the period has drug on and they are destined to continue to diversify as the Devonian transitions into the Carboniferous, not every group will be so lucky, however, for as the Devonian comes to an end disaster strikes once again.
About 358.9 million years ago an event known as the Hangenberg Extinction occurred marking the end of the Devonain period. The event itself was caused by a swift drop in the global temperature; this would also signal the beginning of the Late Paleozoic Ice Age, an event destined to last for millions of years. This quickly had a domino effect and while not as many genera died out overall as the Kellwasser Extinction the effect on life was arguably even more significant. Notably entire families were wiped out in short order, perhaps the most significant loss would be the placoderms. Throughout the Devonian, placoderms had been the dominant fish in the sea, with many genera from the scum sucking Bothriolepis, to the terrifying Dunkleosteus having evolved to fill many different ecological niches and yet they were ultimately unable to adapt to the new world being formed around them.
Having already declined thanks to the Kellwasser Event and repeated smaller extinctions that dotted the late Devonain, it is perhaps no surprise that the Hangenberg Event was simply the straw that broke the camel's back and finally brought an end to this great lineage of fish. Placoderms were not the only fish to suffer from the Hangenberg Extinction, the lobe finned fish also declined significantly in number, while they would survive they would never recover their Devonian Diversity. Reefs systems having already grown rare due to the Kellwasser Event, were also affected heavily with stone corals seeing heavy losses and yet they were lucky compared to the Stromatoporoid sponges which were completely wiped out, this would also lead to heavy losses for creatures like the trilobites. Despite the devastation, however, some creatures managed to pull through and now with many empty niches left open they could begin to diversify. The ray finned fish now no longer as restricted by the lobe finned fish could begin to diversify and thrive in new niches that had now opened up.
They weren't the only ones, the Chondrichthyes survived the Hangenberg Extinction without much issue and having already been successful in the Devonian, were able to take the niche of apex predator, opening the door for many new and terrifying forms. Life on land meanwhile was relatively unaffected by the Hangenberg Extinction and now they were destined to truly begin to diversify. As the forests continued to grow and the Carboniferous began, the Tetrapodomorphs were set to truly master life on land in a way they had never done before.
Trivia
Let's get the elephant out of the room first, the sizes of Dunkleosteus and Titanichthys. Many of you are likely aware of the paper that downsized both of these placoderms and was a pretty big deal when first announced, however, the reason I didn't go with the sizes shown there, was because the reality is the paper is not an end all be all to the actual size of Dunkleosteus or Titanichthys, both of these fish are known from partial remains meaning any estimates are just that estimates, plus the paper itself has seen its fair share of criticism from some. In saying all of this the paper isn't a bad one by any means, but I would caution taking it as the end all be all. I ultimately went with an in between size between the old orca sized estimates and the estimates shown in the paper to try and stay in the middle but of course this is subject to become outdated, if we ever find a complete specimen of Dunkleosteus or Titanichthys that is simply the nature of Paleontology.
The title is an obvious reference to how this chapter and the last one are closely connected.
I was considering having the great white shark sized species of Ctenacanthus make an actual appearance alongside the smaller species, however, I ultimately didn't go through with the idea as I couldn't find a good way to integrate it into the story.
I originally had planned for a scene where the Dunkleosteus eats a Ctenacanthus, but this was removed as I couldn't find a good spot to integrate it.
I also planned on a scene where the Ctenacanthus gets eaten by the Hyneria, but this was cut for the same reason.
A final scene that was cut, was one where our Hynerpeton is chased by a Ctenacanthus.
The Ctenacanthus and the Dunkleostus feeding on the dead Titanichthys at the start is based on modern feedings of dead whales by sharks.
The Dunkleosteus hunting the Titanichthys, is based on bite marks that have been found on Titanichthys fossils. I have also always found it odd that many documentaries with Dunkleosteus have it hunting small fish, despite the fact that they lived alongside larger fish hence the inclusion of Titanichthys.
I was origionally going to have the land portions set in Greenland and use Ichthyostega and Acanthostega, however, I changed my mind, as the areas that these two genera have been found in don't seem to have any other creatures to my knowledge, plus Hynerpeton seems better adpated to life on land anyways which makes it a better continuation so to speak of the tetrapodmomorphs story.
Thats a wrap for this chapter and now we are about to get into a favorite period of many the Carboniferous.